Most railway track is at grade and supported on ties constructed of wood, concrete or other materials. At grade track is usually supported upon ballast which can include crushed stone or other suitable materials to support the weight of high capacity trains. Because of the high weight of a fully loaded train and the dynamics of the track as the train is moving at speed along a track section, certain amounts of vibration and movement of the individual rails may occur. In this regard the at-grade ballast and the compressibility of the railway ties provides an acceptable support for the railway vehicle. However when a rail vehicle is required to cross an area where ground support may be inadequate, such as a stream or river crossing, a more rigid structure such as a bridge is required. A superstructure such as a bridge may often support the rails in a more rigid fashion than the at-grade crossing. But bridges are generally of a short length, and the lack of ballast does not effect the operation of the vehicle. In addition, some bridges and other structures because of related considerations may be required to be movable, such as drawbridges or turntables. In such movable structures the rail is temporarily broken so that the section of bridge can be moved. It has been common practice in many sections of railway track to utilize track circuits to monitor the integrity of the rails. Circuits operating in such a fashion are often referred to as "broken rail" detection circuits. In such circuits a rail current in the rails is monitored and, if the continuity is interrupted, this condition can be considered an indication that the rail has become broken or separated, possibly at a joint or other location. Such broken rail circuits are often incorporated along with existing track circuits which indicate the occupation of the track by a rail vehicle. Some railway track occupancy circuits also provide a portion of rail integrity monitoring. In addition, many superstructures such as bridges that are movable may include electrical or mechanical interlocks which provide an indication that the bridge or structure is opened, or closed such that the rails are properly mated at sections where they have been separated for the purpose of opening the structure.
Generally devices that detect structural misalignment on a movable structure are made to interlock when the bridge or structure is returned to its normal position. Such interlocking may be designed only to detect crude misalignment while remaining insensitive to the normal vibration and strain associated with full loaded trains operating over the respective section of track. Such devices are of a nature so as to be relatively insensitive to the presence or absence of rail vehicles on the section of track in which they are interlocked.
However, because broken rail detectors have traditionally been utilized at grade, the integrity of the grade has been assumed as the earth does not usually move without still providing adequate support for the rails. However, superstructures such as bridges can deteriorate or sustain damage that makes the bridge have questionable structural integrity without breaking the rail. In addition, track circuit broken-rail detection will usually operate in a mode to monitor track integrity while they are unoccupied. This is because such circuits depend upon rail current, and such current is shunted when a rail vehicle occupies a section of track.
Therefore it would be desirable to have a measure of hazard prediction for both the integrity of the rail and the supporting structure that would operate during both occupied and unoccupied track conditions. It would also be desirable if such system could in fact operate over a period of time so as to detect deterioration in the rail and support structure which may occur gradually. The system should also indicate when it is desirable to inspect the specific section of track prior to an adverse condition occurring. In addition, the system should indicate spontaneous structural damage due to high impact such as wrecks.